The present disclosure relates to industrial process control or monitoring systems. More specifically, the present disclosure relates to process variable transmitters or sensing devices of the type which use averaging pitot tube (APT) probes or other primary sensing probes to measure a process variable of an industrial process.
In industrial settings, control systems are used to monitor and control industrial processes, and the like. Typically, the control system that performs these functions uses field devices distributed at key locations in the industrial process and coupled to control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes.
Some field devices include a transducer which couples to the process fluid. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, actuators, solenoids, indicator lights, and others.
Field devices, such as process variable sensors used in industrial processes can be installed in the field on pipelines, tanks and other industrial process equipment. One type of process variable sensor is a flow meter that can measure a rate of fluid flow, for example. One type of flow meter, which employs an averaging pitot tube (APT) primary element, is a popular device for flow measurement because of its ability to be inserted into and retracted from a flow line, its low pressure loss, relatively low cost and reliable performance. The APT primary element senses and averages pressures from multiple locations across a pipeline through which a process fluid is traveling. This average pressure is then used in conjunction with flow theory and experimentally determined quantities to provide a flow measurement for the fluid. One type of APT is the Annubar® APT available from Emerson Process Management™. At least for Annubar® type APTs, it is preferred that the APT span the process pipeline such that multiple samples across a section of the pipeline can be averaged to account for variations in flow across the section.
Some types of APT primary elements for differential pressure flow meters can be installed in a process pipe while a process is in service. To assure APT primary element longevity and accuracy, the tip of the APT primary element must firmly seat against the opposite side of the process piping from which it is mounted. If an APT primary element is not inserted fully, it will vibrate due to vortex shedding and send a poor signal to the secondary element (e.g., the transmitter) or break apart causing damage downstream. If the APT primary element is over-inserted, it can buckle resulting in the same problems as not bottoming-poor signal, breakage, and damage to the piping.
A commonly used method of installing an APT primary element into process piping uses a marking on the insert/retract control rods. In an attempt to ensure proper tip APT tip bottoming against the opposite interior wall of the process piping is to insert the APT primary element until a marking on the insert/retract rods nears the gear boxes. used to control insertion. Once the marking nears the gear box, the installer is required to “feel” when the crank handle on the gear box becomes more difficult to turn. The added difficulty to turn the gear box handle is the installer's indication that the tip has bottomed on the opposite side of the pipe. However, it can be extreme difficulty to “feel” when the tip bottoms against the opposite wall. This is because the gear boxes used to insert the APT primary element provide such a mechanical advantage that the additional force of the tip bottoming is negligible compared to the other installation forces of packing gland resistance and process pressure. Not being able to easily tell the difference between a bottomed and un-bottomed tip, it is easy for the installer to either not bottom the APT primary element, or to overload and damage the APT primary element. This results in a failure rate of the APT primary element, sometimes as high as five percent, due to poor installations.
The discussion above is merely provided for general background infoimation and is not intended to be used as an aid in determining the scope of the claimed subject matter.